[Hersbird]

Originally Posted by aardvarcus

I also have a similar GM truck with 4:10 gears.
Is your tow/haul button pushed in? I don't think your transmission is shifting into overdrive, or the gearing is a lot lower than 4.10. Start from a dead stop and count the number of shifts it makes.
You stated 2200 - 2400 at highway speed, I am assuming that is your mentioned 90kph, or about 56 mph. If the speedometer in a GM truck with factory diameter tires reads 56, your are probably actually doing about 51 mph. (Your 265/70R16 is roughly the same diameter as factory 245/75R16.)
51 mph is 4488 feet/minute, and with 31” tires you will go about 8.111 feet per revolution, thus your tires spin 553 times in a minute. With 4:10s in the back, that means your driveshaft is spinning 2268 times a minute. With your transmission in direct drive (3rd), that would mean your engine is also spinning about 2268, so right in the middle of your 2200-2400 range. Your truck should not be in direct drive, it should shift into overdrive (4th) and have a 0.70:1 ratio, dripping your rpms to 1588, which would be about right.
Either that, or someone put rock crawler type gears in it, around a 5:86 ratio to get those rpms at that speed.

See the math has been done, you are in 3rd gear with those RPMs, not 4th. No way somebody swapped in 5.86:1 gears.